EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT 99 



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Fig. 12. Suggested Phyletic History of T-vo Types of Complex Molars. [As 

 in Osborn's diagram, the solid black dots represent the cusps of the upper 

 molars, the circles, those of the lower molars.] 1 to 6, Phyletic history of the 

 " Tritubercular " type; a to d, Phyletic history of the " Triconodont " type; 

 e,f, From the brachyodont Triconodont stage to the bilobed hypsodont type of 

 molar. 



A, B, C, E and G compare with A, B, C, E and G in Osborn's diagram, fig. 

 11 ; ^, Dryolestes type, Atlantosaurus beds (? Upper Jurassic); 5 and 6, Proto- 

 lambda or Pediomys type, Laramie beds (Upper Cretaceous); d, Triconodon 

 type, Atlantosaurus beds ( ? Upper Jurassic) ; f Palceolagus type, White River 

 beds (Oligocene). 



